1. Field of the Invention
The present invention relates generally to optical data storage systems, and more particularly to methods and apparatus for processing data signals from optical storage media.
2. Description of the Related Art
Electronic data storage media are ubiquitous in technologies that use computers. Commonly, data storage media are configured as data storage disks, and data is stored on the disks in closely spaced, substantially concentric circular or spiralled data tracks. More particularly, data is stored on a disk by rapidly rotating the disk under a data storage head, and the data storage head magnetically or optically alters characteristics of material on the disk surface along one or more tracks such that the altered areas of the disk surface represent the data to be stored. Then, to retrieve the data from the disk, the disk is rotated under a data read head. As the data tracks pass underneath the read head, the read head detects the alterations in the disk surface and, consequently, "reads" the stored data for output of the data. A read head may be associated with more than one read sensor to simultaneously read data on a plurality of tracks.
It may readily be understood that the data storage capacity of a disk can be increased by decreasing the spacing between altered segments on the same data track, and by decreasing the spacing between adjacent data tracks. It happens, however, that when the spacing between tracks is decreased to increase the storage capacity of the disk, the resolution of the main sensor of a data read head may be insufficient to read data only in the track of interest. In such a case, the main sensor of a data read head may undesirably read information contained in tracks that are adjacent to the track of interest.
In other words, the readback signals from data read heads, which are intended to be representative only of data stored in a particular track, may undesirably contain information stored on adjacent tracks. Essentially, data stored on the tracks that are immediately adjacent to the track of interest interfere with the data read signal from the track of interest. Such track-to-track interference is referred to as cross-talk.
Not surprisingly, devices have been introduced for cancelling cross-talk in readback signals. For example, U.S. Pat. No. 5,181,161 to Hirose et al. effects cross-talk cancellation by reading data on three adjacent tracks with two side sensors and one center sensor. Then, the signals from the two side sensors are modified and subtracted from the center sensor signal. In devices such as that disclosed in Hirose et al., the side sensor signals are typically modified by aligning the signals in time, equalizing the signals, and adjusting the gain of the signals.
Aligning the signals in time is required because the positioning of the center sensor relative to the side sensors may be such that the center sensor detects side track cross-talk data at some time before or after the side sensors detect the same dam. For this reason, prior to subtracting the side sensor signals from the center sensor signal, the signals must be aligned in time.
Signal equalization in some systems, and particularly in magnetic disk systems, is required because the sensors in such systems typically have a frequency response to off-track signals that is different from their frequency response to on-track signals. More specifically, the center sensor in such systems has a frequency response when reading data on the center track that is different from its frequency response to data on the side tracks. Likewise, the side sensors of such systems have frequency responses when reading data on their respective side tracks that is different from their frequency response to data on the center track. Consequently, in systems which employ sensors the frequency response of which varies depending on whether the data being read is on-track or off-track, the side sensor signals must be equalized. In other words, in many systems, prior to subtracting the side sensor signals from the center sensor signal, the side sensor signals must be modified to account for the different frequency response of the sensors to off-track signals.
Once alignment and equalization have been accomplished, the amplitude of the signals from the side sensors must be reduced in proportion to the contribution of data from the side tracks to the center sensor signal. Stated differently, side sensor signals that are representative of data on the side tracks are much stronger than the side track data contributions to the center sensor signal. Consequently, the aligned, equalized side sensor signals must be reduced in amplitude, and then subtracted from the center sensor signal, to thereby cancel unwanted cross-talk.
U.S. Pat. No. 5,166,914 to Shimada et al. and U.S. Pat. No. 5,181,161 to Hirose et al. are examples of apparatus directed to canceling cross-talk in optical disk systems, and both Shimada et al. and Hirose et al. borrow the signal processing considerations discussed above for magnetic disk systems. For example, Hirose et al. performs all three of the alignment, equalization, and gain adjustment steps discussed above, by using multi-tap transversal filters.
Taking Hirose et al. as an example, a transversal filter is allocated to each track, i.e., a center transversal filter is allocated to a center track and side transversal filters are allocated to respective side tracks. Alignment, equalization, and gain adjustment are performed by each transversal filter on its respective signal, prior to subtracting the side sensor signals from the main sensor signal.
The transversal filters of Hirose et al. function by solving equations that use upwards of ten coefficients which must be adjusted for, among other things, variations in disk linear velocity. Adapting the filter coefficients to variations in disk linear velocity is relatively time-consuming. Consequently, while Hirose et al. is effective for constant linear velocity applications, e.g., large block transfer or sequential access optical devices such as CDs or videodisks, its adaptive filter technique is too slow for random access optical disk data storage applications, in which the disk linear velocity can change rapidly.
We have discovered, however, that the frequency response of an optical read sensor to center track data is substantially the same as the sensor's frequency response to side track data, thereby obviating the need to equalize side sensor signals independently of the center sensor signal.
Accordingly, it is an object of the present invention to provide a cross-talk canceler for use in random access optical disk data storage systems. Another object of the present invention is to provide a cross-talk canceler, for use in optical disk systems, that does not require the use of transversal filtering techniques. Yet another object of the present invention is to provide a cross-talk canceler for use in optical disk systems that is easy to use and cost-effective.